In recent years, the pressure in vehicle engines during combustion tends to increase because of an increased output and improved fuel economy of the engines. Consequently, the required voltage of spark plugs installed in the engines tends to increase during ignition. The higher the required voltage of the spark plugs during ignition, the more easily electrodes of the spark plugs erode.
In a conventional technique to reduce the erosion of electrodes of a spark plug, an end of a core material of a center electrode is coated with a material having a thermal expansion coefficient lower than that of the core material (see Japanese Unexamined Patent Application Publication No. 2015-82355).
However, since such a spark plug uses a material with which the core material of the center electrode is coated, the manufacturing cost of the spark plug increases in some cases. For this reason, there is a need for a technique that enables the erosion of the electrodes to be reduced regardless of the material of the electrodes.
The present invention has been accomplished to address the above problem and can be achieved as the following aspects.